Many personal, commercial, and industrial structures and devices are made from composite structures because composite structures can have high strength and/or stiffness at a low weight. One composite structure configuration, which is present in a variety of applications including unmanned aerial vehicles, includes a lightweight core (e.g., a foam core, balsa core, honeycomb aramid core, and the like) and an overlying composite skin, such as a carbon fiber laminate. The composite skin can include any suitable number of composite plies. Each of the composite plies typically includes composite fibers embedded in a matrix. The composite plies in the composite skin can be oriented to orient the composite fibers in suitable directions. Composite structure can be very strong and very light, but may also be susceptible to in-service damage (e.g., impact damage). In many instances, a damaged composite structure many not show obvious visible signs of damage. Additionally, in many instances, a composite structure many not show obvious visible signs of manufacturing defects (e.g., voids, delaminated areas, resin starved areas).
Because composite structures may not show obvious visible signs of damage and/or manufacturing defect, a variety of inspection methods have been employed to detect the damage and/or manufacturing defect. Common forms of damage include the development of voids, delamination or debond, buckling, and cracks in either (or both of) composite plies and/or composite core. Existing inspection methods include tap testing, ultrasound testing, and radiographic means. Tap testing can include systematic tapping a surface of a composite structure and searching for damage by listening for dead sounds or resonant sounds. Ultrasonic testing can include using an ultrasound scanner that emits ultrasonic pulses and processes resulting return ultrasonic pulses to detect faults in a composite skin or in a composite core. Existing composite inspection methods, however, may have drawbacks in terms of labor, materials (e.g., ultrasound gel) and the potential for operator error. Thus, improved inspection methods for detecting damage and/or manufacturing defects in composite structures are of interest.